1. Field of the Invention
The present invention relates to a liquid ejection apparatus incorporating a supplying liquid circulation system for circulating liquid in a liquid ejecting head and the liquid processing method thereof. In particular, the present invention is suitable as an ink jet apparatus using a full line-type ink jet head in which ejection ports are arranged over the entire width of a printing medium.
The term “print” described in the Specification includes, in addition to a case where significant information (e.g., characters, graphic) is formed, variety of cases such as a case where an image, marking, or pattern is formed on a printing medium or a case where the printing medium is processed (e.g., etching), regardless of the significance or non-significance and regardless of whether or not the information is elicited so as to be visually recognized by a person.
The term “printing medium” includes not only a paper used in a general print apparatus but also materials (e.g., cloth, resin film, metal plate, glass, ceramics, wood, leather) that can accept liquid and materials having a three-dimensional shape other than a sheet-like shape (e.g., sphere, cylindrical body).
The term “liquid” should be widely interpreted as in the case of the definition of the above term “print” and includes any liquids used for printing such as liquid applied to a printing medium to be used for the formation of an image, marking, pattern or the like, liquid for the processing of a printing medium (e.g., etching), or liquid for the processing of ink (e.g., liquid that can be used so that color material in ink applied to a printing medium has coagulation or encapsulation).
2. Description of the Related Art
In an ink jet print apparatus, ink is ejected from an ink jet head (hereinafter also referred to as “print head”) so that the ink is applied to a printing medium for printing, for example. The ink jet print apparatus is advantageous in that the print head can have a compact body in an easy manner, a high-definition image can be printed with a high speed, the running cost is low, the non-impact method reduces noise, and inks having a number of colors are used to print a color image in an easy manner, for example. The so-called full line-type ink jet head is particularly advantageous because a number of ejection ports are arranged over the entire width of the image formation region of a printing medium so that the ejection ports can eject ink simultaneously to form an image with a higher speed. The full line-type print head includes a number of ejection ports arranged in a longitudinal direction and thus a common liquid chamber for storing ink supplied to the respective ejection ports also has a long shape accordingly.
The full line-type print head as described above also has a number of heaters for ejecting ink. This causes a tendency where the ink in a common liquid chamber is heated by a heater to have a high temperature. To prevent this, a technique has been known in which the space in a common liquid chamber of a print head and a sub tank for storing ink supplied to the common liquid chamber are used as a circulation passage so that a pump provided to the passage is used to circulate ink, thus allowing the ink in the sub tank to be circulated in the common liquid chamber. Such a circulation of ink prevents the ink from having a high temperature to suppress the temperature increase of the print head.
The operation for circulating ink as described above also has, in addition to the purpose for suppressing the temperature increase of ink, another purpose for exhausting bubbles accumulated in the common liquid chamber to outside, for example.
FIG. 9 is a cross-sectional view schematically showing a supplying ink circulation system disclosed in Japanese Patent Application Laid-Open No. 11-179932(1999).
As shown in FIG. 9, the supplying ink circulation system 150 has the ink jet head 101, the subtank 103 temporally storing ink to be supplied to the ink jet head 101, and the main tank 102 for storing ink. The supplying ink circulation system 150 is used by being provided to an ink jet printer (not shown).
The ink jet head 101 includes a plurality of ejection ports 101a for ejecting ink, and one common liquid chamber 126 for storing ink to be supplied to the respective ejection ports 101a. At a position at which the ink jet head 101 is opposed to a port opening surface, the cap 108 is provided for receiving ink pushed out of the ejection port 101a. 
The subtank 103 includes the first tank 103a and the second tank 103b. The first and second tanks 103a and 103b are divided to have an enclosed space, respectively. The first tank 103a and the second tank 103b store ink while including therein a predetermined amount of air buffer. The existence of air buffer left in this manner absorbs the fluctuation of the flow rate of ink caused when the ink is circulated.
The first tank 103a has, at the upper face thereof, the air communication passage 134 for communicating air in the tank. The air communication passage 134 is attached with the air communication valve 106d for opening or closing this communication passage.
The main tank 102 has an ink cartridge-like shape so that the main tank 102 can be exchanged with a new one in an ink jet printer (not shown) and stores therein ink having a predetermined color.
The respective components as described above are appropriately connected by tube members. As a result, the ink jet printer can be operated with “ink supply mode”, “ink circulation mode”, “ink eject mode” or the like. Among these operations, the “ink circulation mode” will be described with regards to the configuration and operation.
In order to circulate ink in the common liquid chamber 126, the common liquid chamber 126 has, at the upstream side and the downstream side, the first passage 132 and the second passage 133 communicated to each other, respectively.
The other end of the first passage 132 is communicated with the second tank 103b of the subtank 103 while the other end of the second passage 133 is communicated with the first tank 103a. The first and second tanks 103a and 103b are communicated to each other by a tube member. As described above, the supplying ink circulation system 150 has one circulation passage by the first passage 132, the second passage 133, and the tube member for communicating the first tank 103a to the second tank 103b. 
The tube member for communicating the first tank 103a to the second tank 103b has, at the intermediate position thereof, the first pump 104 for moving ink in the first tank 103a into the second tank 103b. This first pump 104 is used to circulate ink.
The cap 108 is communicated with the collection passage 135 for collecting ink received by the cap 108. The other end of the collection passage 135 is communicated with the space in the first tank 103a of the subtank 103. The collection passage 135 includes the filter 152 for capturing foreign matters in ink and the second pump 109 for sucking ink from the cap 108.
The supplying ink circulation system 150 structured as described above is driven with the “ink circulation mode” as described below.
When the first pump 104 is driven while the air communication valve 106d being closed, ink in the first tank 103a is flowed into the second tank 103b. As a result, the ink in the second tank 103b is pressurized and is flowed via the first passage 132 to the common liquid chamber 126 (see the direction shown by the arrow in the drawing). In accordance with this, ink in the common liquid chamber 126 is partially pushed out into the second passage 133 and is returned to the first tank 103a via the second passage 133. The ink left in the common liquid chamber 126 is partially pushed out of the ejection port 101a and is received by the cap 108.
Then, the second pump 109 is driven in synchronization with the first pump 104 so that the ink received by the cap 108 is returned via the collection passage 135 to the first tank 103a. 
The following section will describe in detail the circulation operation as described above.
First, immediately after the start of the circulation operation, the first pump 104 is driven to flow ink into the second tank 103b and the space in the second tank 103b is pressurized while the air buffer therein being compressed. The pressurization of the second tank 103b in this manner pushes the ink in the tank toward the common liquid chamber 126. On the other hand, ink in the first tank 103a is sucked toward the second tank 103b and thus the tank has therein a negative pressure to inflate the air buffer. In the situation immediately after the start of the circulation operation as described above, the pressures in subtank 103 and in the common liquid chamber 126 are not stabilized yet and thus a relatively large amount of ink is pushed out of the ejection port 101a. When a filter (not shown) for cleaning ink is provided at the side of the second passage 133 in FIG. 9 in particular, a larger amount of ink is pushed out because the space in the common liquid chamber 126 tends to be pressurized due to the influence by the pressure loss of this filter.
When a certain period of time has passed since the start of the circulation operation, the pressures in the subtank 103 and in the common liquid chamber 126 are stabilized. Specifically, the inflation or contraction of the air buffer is stopped and the amount of ink pushed out of the ejection port 101a is also reduced, thus causing the amount of ink flowing into the subtank 103 to be the same as that of ink flowing in the first pump 104.
However, the circulation system as described above causes the subtank to be closed while the common liquid chamber being communicated with air via the ejection port even when the circulation operation is performed in the stabilized condition, thus causing the differential pressure between the common liquid chamber and the subtank. Due to this reason, the ink circulation operation may not be stopped in some cases, even when the pump is stopped. As a result, the common liquid chamber has therein a negative pressure. This has caused a case in which the negative pressure having a magnitude that exceeds an ink meniscus retention force in the ejection port causes air to be sucked via the ejection port. When the air sucked via the ejection port is collected as bubbles in the common liquid chamber, the ejection may not be provided to a correct manner.
The air suction phenomenon as described above tends to be caused as the pump has a larger flow rate or as the air buffer in the subtank has a larger capacity. The air suction phenomenon also tends to be caused when the exhaust side of the common liquid chamber has a filter and the filter has a larger pressure coefficient. Specifically, the prevention of the air suction as described above is desirable because it improves the freedom in the selection of a pump or a filter or the freedom in the selection of the setting of an air buffer.
The ink circulation operation may have, in addition to a defect caused by the air suction as described above, a defect in which the space in the common liquid chamber is pressurized immediately after the start of the circulation operation to cause the ink to be pushed out of the ejection port. The ink pushed out as described above is not particularly problematic in the configuration as shown in FIG. 9 in which the pushed-out ink is again returned to the subtank 103. However, the pushed-out ink is a problem in a configuration in which the pushed-out ink is collected by an independent waste ink collection tank.